Tea tree oil (TTO), the essential oil of Melaleuca alternifolia L., is one of the most popular agents used in cosmetics. This study aimed to explore the correlations of components in TTO with its anti-acne activities and skin irritation. TTOs were isolated from the leaves (TTO-L), twigs, and branches of M. alternifolia by steam distillation, and the components analyzed by gas chromatography-mass spectrometry. Results showed that components of TTO-L satisfied the International Organization for Standardization (ISO) 4730 guidelines. TTO-L and its components, terpinen-4-ol, terpinolene, α-terpinene, and α-terpineol, had strong inhibitory activities against Propionibacterium acnes and Staphylococcus aureus. Moreover, six TTO formulas (DF-TTO) were designed according to ISO 4730 guidelines by adjusting the proportions of these four active components. All DF-TTO formulas showed a positive correlation between terpinen-4-ol concentration and anti-P. acnes activity. In the skin irritation assay, TTO-L, terpinen-4-ol, and 1,8-cineole did not cause significant skin irritation at 2% per site. In conclusion, terpinen-4-ol is the major active component responsible for TTO's antibacterial efficacy, while minor components in TTO also contributed to its efficacy. Moreover, we suggest that a concentration less than 5% is more suitable and safer for treating acne than higher concentrations.
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